1. Field of the Invention
The present invention relates to an electric power steering apparatus which is capable of correcting operation reference values of a direction inhibiting circuit in response to the offset of a center value of the steering torque sensor.
2. Description of the Related Art
An electric power steering apparatuses having a steering torque sensor composed of a differential transformer is known in which a steering torque in the clockwise or counterclockwise direction produced during operation of a steering wheel is detected in the form of an electric differential output from the differential transformer such that intensity and polarity of the steering torque are represented by absolute value and polarity (.+-.) of the differential output, respectively.
According to another known electric power steering apparatus, the steering torque sensor includes an operational amplifier driven by a single power supply of 5V with an imaginary ground of 2.5V. When the differential output from the differential transformer is 0 (steering torque is zero), the operational amplifier (steering torque sensor) generates a detection output of 2.5V. For the differential outputs of negative polarity (steering torques in the counterclockwise direction), the operational amplifier generates detection outputs in the range of 0 to 2.5V while for the differential outputs of positive polarity (steering torques in the clockwise direction), detection outputs in the range of 2.5 to 5.0V are generated from the operation amplifier.
Thus, the steering torque sensor has a neutral point indicative of a condition in which no steering torque is applied to the steering wheel (i.e., steering torque is 0), and the outputs of the operational amplifier have a center value of 2.5V.
When the output (center value) of the operational amplifier corresponding to the neutral point (steering torque=0) is varied either increasingly or decreasingly from 2.5V due to some reasons such as aging (or variation with time) of the differential transformer constituting the steering torque sensor, a center value compensating circuit is used to set the varied output (center value of, for example, 2.6V or 2.4V) as a new center value to thereby apply power assist to the left and right steerable front wheels in a balanced condition in response to steering torques applied to the steering wheel in the clockwise and counterclockwise directions.
An electric power steering apparatus having a direction inhibiting circuit is also known in which a direction signal contained in a motor control signal output from a control system for controllably driving an electric motor and a direction signal output from the direction inhibit circuit are judged for their agreement or matching, and when these direction signal do not match with each other, power assist by the electric motor in a direction opposite to the steering direction is inhibited.
The direction inhibit circuit has two operation reference values corresponding to detection outputs in the clockwise and counterclockwise directions, respectively, from the torque sensor. When the detection output in the clockwise direction exceeds the operation reference value in the counterclockwise direction, power assist by the electric motor in the counterclockwise direction is inhibited. Conversely, a detection output in the counterclockwise direction exceeding the operation reference value in the clockwise direction inhibits power assist by the electric motor in the clockwise direction. With this arrangement, when the direction signal produced from the direction inhibit circuit is not in match with the direction signal of the motor control signal output from the control system due to, for example, a trouble caused in a CPU (microprocessor) constituting the control system, power assist by the electric motor based on the motor control signal output from the control system is inhibited.
Since power assist by the electric motor acting in a direction opposite to the driver's intention, which may occur when the electric motor is driven to generate power assist in the counterclockwise direction in despite of the driver's actuation of the steering wheel in the clockwise direction, is inhibited, an unusual or strange behavior of the vehicle does not take place.
In the conventional electric power steering apparatus, adjustment of the center value (neutral point) of the steering torque sensor is performed either before or when the steering torque sensor is assembled in the vehicle. However, the adjustment is tedious and time-consuming because a mechanical adjustment process must be performed to find out the neutral point of the steering torque sensor (where the detection output from the steering torque sensor is 2.5V) before an input shaft of the steering shaft and a torsion bar are pinned together.
To deal with this problem, an electric adjustment process is developed and used in practice to electrically adjust the center value (neutral point) of the steering torque. According to the known electric adjustment process, an electronic control unit (ECU) having a memory storing therein preset possible values for the center value is used so that adjustment of the center value can be performed automatically by a microprocessor (CPU) incorporated in the ECU. With this electrical adjustment process, the center value adjustment is rendered simple and time-saving.
The electric adjustment process has a drawback, however, that since the operation reference value of the direction inhibiting circuit, which is provided to deal with uncontrollable running of the microprocessor (CPU), has a constant value such as 30 kg.multidot.cm. in steering torque for both the left and right directions, an offset of the center value greater than 10 kg.multidot.cm cannot be used in practice. To realize the center value offset less than 10 kg.multidot.cm, use of an expensive steering torque sensor is inevitable; if not so, setting of the center value will encounter a great difficulty incurring additional cost.
FIG. 10 of the accompanying drawings is a table showing the relationship between the steering torque and the torque detection output taken with respect to the offset of the center value of the torque sensor incorporated in a conventional electric power steering apparatus.
As shown in FIG. 10, the torque center value (neutral point) C.sub.O1, the left operation reference value K.sub.L for inhibiting power assist in the left direction of the direction inhibiting circuit, and the right operation reference value K.sub.R for inhibiting power assist of the right direction are set to be a nominal value of 2.5V, a value of 2.7V, and a value of 2.3V, respectively.
Under such conditions, the steering torque is zero (0) and the torque detection output T.sub.O is 2.5V which is equal to the center reference value C.sub.O1. When the right steering torque (actuation of the steering wheel in clockwise direction) T.sub.R increases to T.sub.R1, the torque detection signal T.sub.ON becomes equal to the left operation reference value K.sub.L (2.7V) of the direction inhibiting circuit with the result that power assist by the electric motor in the left direction corresponding to the actuation of the steering wheel in the counterclockwise direction is inhibited.
Conversely, when the left steering torque (actuation of the steering wheel in the counterclockwise direction) T.sub.L increases from zero (0) to T.sub.L1, the torque detection output T.sub.ON becomes equal to the right operation reference value K.sub.R (2.3V) of the direction inhibiting circuit with the result that power assist by the electric motor in the right direction corresponding to the actuation of the steering wheel in the clockwise direction is inhibited.
Accordingly, in the case where a motor control signal tending to drive or rotate the electric motor in a direction opposite to the direction of a steering torque produced by actuating the steering wheel (a right steering torque T.sub.R, for example) is output due to, for example, uncontrollable running of the microprocessor (CPU) in the control system which assists the electric motor on the basis of the torque detection output T.sub.O, the direction inhibiting circuit inhibits power assist by the electric motor when the steering torque reaches the operation reference, value (the left operation reference value K.sub.L, for example)
Since the right and left operation reference values K.sub.R and K.sub.L are set to be 2.7V and 2.3V, respectively, and since they K.sub.R, K.sub.L have the same offset (0.2V) with respect to the center value C.sub.O1 (2.5V), power assist by the electric motor is inhibited in a balanced or symmetrical condition with respect to steering torques (T.sub.L1, T.sub.R1, .vertline.T.sub.L1 .vertline.=.vertline.T.sub.R1 .vertline. ) increasing in the left and right directions from the zero (0) steering torque point.
In the case where the center value (neutral point) of the steering torque sensor is varied from C.sub.O1 (2.5V) to C.sub.O2 (2.6V), torque detection outputs T.sub.O corresponding to the right steering torques T.sub.R and left steering torques T.sub.L are represented by the characteristic line T.sub.OH. When the right steering torque T.sub.R becomes equal to T.sub.R2 (T.sub.R1 -.DELTA.T), the torque detection output T.sub.OH reaches the left operation reference value K.sub.L (2.7V) whereupon the direction inhibiting circuit is activated to inhibit power assist by the electric motor in the left direction.
Also, when the left steering torque T.sub.L is T.sub.L2 (T.sub.L1 +.DELTA.T), the torque detection output T.sub.OH becomes equal to the right operation reference value K.sub.R (2.3) whereupon the direction inhibit circuit is activated to inhibit power assist by the electric motor in the right direction.
As is apparent from FIG. 10, the left operation reference value K.sub.L (2.7V) in the direction inhibit circuit is reached when the right steering torque T.sub.R is T.sub.R2 (T.sub.R1 -.DELTA.T), whereas the right operation reference value K.sub.R (2.3V) in the direction inhibit circuit is not reached until when the left steering torque T.sub.L becomes equal to T.sub.L2 (T.sub.L1 +.DELTA.T). Since T.sub.L1 and T.sub.R1 have the same absolute value ( .vertline.T.sub.L1 .vertline.=.vertline.T.sub.R1 .vertline. ), the absolute value of T.sub.L2 is greater than the absolute value of T.sub.R2 by 2.DELTA.T. As a result, left and right steering torques (T.sub.L, T.sub.R) at which the direction inhibiting circuit is activated are in an ill-balanced condition.
If two microprocessors (CPUs) are used to formed the direction inhibiting circuit, a great offset of the center value such as 10 kg.multidot.cm or more becomes available by the use of software processing Use of the two microprocessors can lower the cost of the steering torque sensor but increases the cost of the direction inhibiting circuit.